1. Field of the Invention
The present invention relates to a valve system for an engine, in which a valve stem for an engine valve is slidably fitted in a guide cylinder provided in a cylinder head of the engine. In addition, a rocker arm is interlocked with a camshaft of the engine to rock with rotation of the camshaft. The rocker arm is interlocked with the valve stem.
2. Description of Background Art
Valve systems of this type have been known, for example, from Japanese Patent Laid-open No. Sho 56-27009 and Japanese Patent Publication No. Hei 1-20290.
In the above-described background art valve system, an upper end of a valve stem is pressed down by a threaded tappet screwed in a rocker arm, with the forward/backward position of the tappet screw being adjustable. With this structure, since the threaded tappet is moved along a circular-arc centered at the fulcrum of the rocker arm, a bending load is applied to the valve stem. This results in the occurrence of local wear, galling, and the like in the guide cylinder provided in the cylinder head to thereby effect the guiding of the valve stem.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a valve system for an engine, which is capable of preventing occurrence of local wear, galling, and the like of a guide cylinder, thereby enhancing the reliability.
To achieve the above object, according to a first aspect of the present invention, a valve system for an engine is provided, in which a valve stem for an engine valve is slidably fitted in a guide cylinder provided in a cylinder head. A rocker arm is interlocked with a camshaft to rock with rotation of the camshaft. The rocker arm is interlocked with the valve stem. The first aspect of the present invention is characterized in that a lifter is interposed between the rocker arm and the valve stem. The lifter is formed into a cylindrical shape with its bottom closed and having a diameter larger than an outside diameter of the valve stem. The lifter is fitted in a lifter housing in such a manner as to be slidable in the axial direction of the valve stem. The lifter housing is fixed to the cylinder head.
With this configuration, a drive force from the rocker arm is applied to the valve stem of the engine valve via the lifter having a cylindrical shape with its bottom closed. Accordingly, a bending load is not applied to the valve stem having a relatively small diameter, so that it is possible to prevent the occurrence of local wear, galling, and the like in the guide cylinder. Furthermore, the lifter has a relatively large diameter. Accordingly, even if a bending load is applied from the rocker arm to the lifter, it is possible to minimize the occurrence of local wear, galling, and the like between the lifter housing and the lifter, and hence to improve the reliability.
According to a second aspect of the present invention, in addition to the configuration of the first aspect of the present invention, the lifter housing is formed into a cylindrical shape to allow the lifter to be slidably fitted therein. The lifter housing has an oil hole opened in an inner surface of the lifter housing. With this configuration, since the lifter can be more smoothly slid in the lifter housing, it is possible to more surely prevent the occurrence of local wear, galling, and the like between the lifter housing and the lifter.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.